Backtesting applies trading rules to historical market data to determine the viability of the idea. When designing a system for automated trading, all rules need to be absolute, with no room for interpretation. The computer cannot make guesses and it has to be told exactly what to do. Traders can take these precise sets of rules and test them on historical data before risking money in live trading. Careful backtesting allows traders to evaluate and fine-tune a trading idea, and to determine the system’s expectancy.